1. Field of the Invention
The present invention relates to a slitter blade assembly comprising a drum-shaped rotary blade and a disk-shaped rotary blade, for cutting off a thin flat workpiece such as a film or the like.
2. Description of the Related Art
Rotary blade assemblies for cutting off thin flat workpieces including films, sheets of paper, metal foils, etc., for example, generally comprise an upper blade and a lower blade which rotate in respective opposite directions while their circumferential edges are being held in sliding contact with each other, for continuously cutting of the thin flat workpiece. The shape of cutting edges on a rotary blade greatly affects the quality of severed surfaces on the workpiece.
Japanese patent publication No. 7-67675 discloses a conventional rotary blade assembly comprising upper and lower circular blades whose cutting edges are progressively beveled away from the companion blades to give severed surfaces a desired shape. The disclosed rotary blade assembly is suitable for cutting off a film having a base of TAC (triacetyl cellulose), for example.
Some films that have been developed in recent years have a base of PEN (polyethylene naphthalate). The PEN has such properties that it cannot easily be ruptured because of high mechanical strength and can easily be stretched. When the conventional rotary blade assembly is applied to the cutting of a film having a PEN base, depending on the beveled edge settings, as shown in FIG. 6 of the accompanying drawings, severed surfaces 3a, 3b of a base 2 which supports an emulsion layer 1 may suffer a crack 4 or a whisker 5, tending to lower the quality of the severed film.
Another conventional rotary blade assembly which is capable of well cutting off a PEN base is revealed in Japanese laid-open patent publication No. 7-272270. As shown in FIG. 7 of the accompanying drawings, the revealed rotary blade assembly has an upper blade 6 including a tapered surface 8 contiguous to a cutting edge 7. When the tapered surface 8 is pressed against an emulsion layer 1 and a base 2 that are placed on a lower blade 9, internal stresses are developed in the emulsion layer 1 and the base 2 under tensile forces prior to the severance of the emulsion layer 1 and the base 2. Thereafter, the emulsion layer 1 and the base 2 are cut off by the cutting edge 7. In this manner, the emulsion layer 1 and the base 2 can be cut off with good severed surfaces 3a, 3b. 
However, since the tapered surface 8 of the upper blade 6 is pressed against the emulsion layer 1 when the emulsion layer 1 and the base 2 are severed, an edge 10 of the upper blade 6 remote from the cutting edge 7 presses the emulsion layer 1, tending to apply a striped mark to the emulsion layer 1.